1. Field of the Invention
The field of the invention relates to systems and methods for providing parallel power in a hybrid-electric vehicle.
2. Background
Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle, and provide better fuel economy than comparable conventional vehicles. This combination offers the extended range and rapid refueling that consumers expect from a conventional vehicle, with a significant portion of the energy and environmental benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles.
A hybrid""s efficiency and emissions depend on the particular combination of subsystems, how these subsystems are integrated into a complete system, and the control strategy that integrates the subsystems. Existing HEVs use complex integration systems, which, while efficient, have not yet proven to be economically feasible. The commercial success of HEVs has been hindered by the economics of producing a complex hybrid power system rather than by the inherent capabilities of the technology. Complexity is a major disadvantage of existing HEV configurations, and has inhibited the acceptance of HEVs in the marketplace.
HEV configurations fall into two basic categories: series and parallel. In a series hybrid, the internal-combustion engine drives a generator that charges the batteries, which power an electric motor. Only the electric motor can directly turn the vehicle""s wheels to propel the vehicle. In contrast, in a parallel hybrid either the engine or the motor can apply torque to the wheels. Both the parallel and the series hybrid can be operated with propulsion power supplied only by the internal-combustion engine. But in a series hybrid, this power is inefficiently applied through the generator and the electric motor. Parallel HEVs do not require a generator, because the motor generates electricity when driven by the engine. Parallel HEVs are thus less complex than series HEVs. Another advantage of the parallel scheme is that a smaller engine, electric motor, and battery pack can be used, because the engine and the motor work together to drive the vehicle.
Turning to series HEVs, an advantage of series configurations is that the internal-combustion engine can be located anywhere in the vehicle because it does not transmit power mechanically to the wheels. This is advantageous for designing the vehicle because the designer has more freedom of choice in determining where the internal combustion engine should be located. In contrast, parallel configurations must connect both the motor and the engine to the drivetrain. This requires the motor and the engine to be in proximity to each other. Though parallel configurations are generally preferred for their flexible power output, the difficulty of packaging both a conventional engine and a conventional electric motor in a drivetrain has been a major disadvantage of existing parallel HEVs.
Another problem with conventional parallel configurations is that they do not allow easy conversion of conventional vehicles to parallel HEVs. For example, a parallel HEV system is disclosed in U.S. Pat. No. 6,083,138 issued to Aoyama et al. (Aoyama). Because of the nature of the system disclosed in Aoyama, however, complex control systems are required to control a variable torque clutch 3, a Continuously Variable Transmission (CVT), the internal combustion engine, and a plurality of motors, one of which is required to power the CVT""s hydraulic pump system. Consequently, a conventional vehicle cannot be easily converted to a parallel HEV using the system disclosed in Aoyama, because the system disclosed in Aoyama is not compatible with a conventional clutch or a conventional transmission. Thus, these components would also need to be replaced. Further, the complex control systems and plurality of motors also need to be installed making conversion of a conventional vehicle using the systems disclosed in Aoyama even more difficult.
An aspect of the present invention involves a system and method for providing parallel power in a hybrid-electric vehicle. The system includes a compact motor coupled to the input shaft of the vehicle""s transmission. Advantageously, the compact motor and the engine use the same drivetrain. Both the compact motor and the engine are able to apply power to the portion of the drivetrain from the transmission to the wheels. Since the motor is compact and does not require a separate drivetrain, the parallel power system can be installed in an otherwise conventional vehicle without packaging difficulties.